Solubilization of organic acids

ABSTRACT

PROCESS FOR PURIFYING RELATIVELY INSOLUBLE ACIDS SUCH AS TEREPHTHALIC BY FORMING A SOLUTION OF THE ACID IN AN AQUEOUS SOLUTION OF WEAK ACID SALTS OF LITHIUM, TERTIARY AMINES, OR TERTIARY ALKYL AMMONIUM COMPOUNDS, FOLLOWED BY TREATING THE SOLUTION TO REDUCE IMPURITIES PRESENT AND RECRYSTALLIZATION OF THE PURIFIED ACID FROM SOLUTION.

United States Patent U5. Cl. 260-514 K 11 Claims ABSTRACT OF THEDISCLOSURE Process for purifying relatively insoluble acids such asterephthalic by forming a solution of the acid in an aqueous solution ofweak acid salts of lithium, tertiary amines, or tertiary alkyl ammoniumcompounds, followed by treating the solution to reduce impuritiespresent and recrystallization of the purified acid from solution.

This application is a continuation of application Ser. No. 564,541,filed July 12, 1966, now abandoned.

This invention relates to a method of increasing the solubility oforganic acids. More particularly, the invention is directed to a processfor increasing the solubility of organic carboxyic acids such asterephthalic acid by use in aqueous solution of weak acid salts oflithium, tertiary amines, or quaternary ammonium compounds.

The commercial importance of organic acids such as terephthalic acid inthe production of polyesters and in many other uses has increasedconsiderably in recent years. For example, an acid such as terephthalicacid is capable of entering into a condensation polymerization withvarious glycols, such as ethylene glycol and the like, to produceresinous compositions which are capable of being formed into valuablefibers and filaments. In order to obtain these products, however, it isnecessary that the organic acids exist in a pure state. Because of thevery low solubility of these organic acids, purification by normalprocedures becomes extremely difiicult. The most widely used procedurepreviously practiced for purification of insoluble organic acids is toconvert the acid to an ester. This has the eifect of not onlyneutralizing the acid, but the esters so produced have a much widerrange of distillation temperatures thereby making physical separationfeasible.

William G. Toland, Jr., in US. Pat. 2,664,440, discloses another priorprocess for purification in which terephthalic acid is reacted with atertiary amine to form a salt which is readily soluble in aqueoussolution. The salt, as such, is crystallized from solution and heated todecompose the salt to its corresponding terephthalic acid and tertiaryamine, but this is a ratther cumbersome and expensive procedure.

It can thus be realized that while the methods of the prior art areeffective in their own right as means for purifying and increasing thesolubility of acids such as terephthalic acid, they tend to beimpractical on a commercial scale. This is due to the expense requiredin maintaining reaction and distillation vessels as well as the extraprocedures necessary to recover the acid as such.

It therefore becomes evident that it is important to find other meansfor purifying and increasing the solubility of organic acids which aresimpler in their operation and more beneficial from an economicstandpoint.

3,746,756 Patented July 17, 1973 Accordingly, it is a primary object ofthis invention to provide a new and improved method for increasing thesolubility of organic acids to make purification more economical. It isa further object of this invention to increase the solubility of organiccarboxylic acids in aqueous solution thereby providing simple and moreeflieient means for purification. Still another object is to permit thedirect recovery of organic carboxylic acids in the form of relativelylarge crystals by crystallization from an aqueous medium.

In general, the objects of this invention are accomplished by mixing theorganic acid with an aqueous solution of a weak acid salt of (a)lithium; (b) a tertiary amine corresponding to the formula:

where the substituents R R and R may be alkyl, aryl,

' or alkylaryl, and X may be any Weak organic or inorganic acid radical;or (c) a quaternary ammonium compound of the formula (R) N+X- whereinthe R radicals represent alkyls of from 1-6 carbon atoms and X- is anyweak organic or inorganic acid radical. Suitable weak organic orinorganic acids which form the anion of the salts of this invention arethose which have pK values of from about 3 to 5. Examples of such weakacids include organic acids such as formic acid, acetic acid, propionicacid, butyric acid, isobutyric acid, succinic acid, lactic acid andp-choro-benzoic acid; and inorganic acids such as tetraboric acid,hydrofluoric acid, selenic acid, and telluric acid.

The amines which form the cation of the salts used in the practice ofthis invention are those having pK values of from about 2.5 to about5.0. Suitable examples of such amines include trialkylamines, such astrimethylamine, triethylamine, tripropylamine, triisopropylamine,alkanolamines such as triethanolamine, trimethylolamine andtriisopropanolamines, and cyclic amines such as N-methyl morpholine,cyclohexyldimethylamine, N-methyl-piperidine and N,N-dimethyl aniline.

Suitable examples of weak acid salts of tertiary amines used in thepractice of this invention are triethylammoniurn acetate,triethylammonium formate, triethanolammonium acetate,trimethanolammonium butyrate, trimethylammonium acetate. N,N-dirnet-hylaniline acetate. Illustrative of the quaternary ammonium salts which maysuccessfully be employed in the present invention are tetrabutylammoniumacetate, tetrapropylammonium acetate, tetramethylammonium hydroxide.Examples illustrative of the lithium salts operative in this inventioninclude lithium acetate, lithium propionate and lithium succinate.Organic coupling solvents such as water miscible alcohols and ketonesmay be advantageously added in an amount of up to about of total volume,e.g. 2 to 50% of total volume, to further increase the solubility of theorganic acid in the aqueous salt solution. Preferred alcohols includemethyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol,tertiary butyl alcohol, n-butyl alcohol, etc. The ketones which arepreferred include particularly acetone and methyl ethyl ketone.

Organic acids so solubilized are contained within the resulting solutionin their original form. Appropriate means are available to removeimpurities such as related aldehydes which are originally present anddissolved with the organic acids in the aqueous solution. For example,strong oxidizers such as permanganate, dichromate, or peracetic acid maybe added to the solution to oxidize any aldehyde to its correspondingacid. For instance, when crude terephthalic acid containing 2.5%p-carboxybenzaldehyde as an impurity is dissolved to saturation inaqueous 2 M lithium acetate at 100 C., and 2.5 g. of potassiumpermanganate per 100 g. of dissolved crude terephthalic acid is added tothe solution, the manganese dioxide formed by permanganate reduction isfiltered oil at 100 C., and the filtrate cooled to room temperature, theterephthalic acid crystals formed should contain less than 100 p.p.m. ofp-carboxybenzaldehyde. Alternatively, aldehydes present in the mixturemay be reacted with reagents such as hydroxyl amine to form an oximewhich may be removed by extraction or recrystallization. In addition togiving high product purity, crystallization of the organic acids fromsolution has the advantage of producing crystals of large size. Anyimpurities occluded or otherwise physically entrapped within such largecrystals can be readily removed by further recrystallization andwashing.

The acid salts of lithium, tertiary amines, or quaternary ammoniumcompounds, may be prepared by any of the accepted procedures, forexample, reacting lithium oxide with acetic acid to produce lithiumacetate.

The salts so produced are dissolved in an aqueous solution to a molarconcentration of from about 0.5 M to 4 M, preferably from 1 molar to 2molar concentration. The solubilization of the organic acid may takeplace in solution having a temperature of from about to 200 C. with apreferred temperature being 70 to 200 C. Appropriate superatmosphericpressure may be used to maintain the solution in liquid phase.

This invention is particularly useful for obtaining fibergradeterephthalic acid. However, in its broad concept, it is similarlyapplicable in the purification of other solid organic carboxylic acidswhich may be aromatic, aliphatic, or cycloaliphatic in nature. Examplesof such aromatic acids are phthalic acid, terephthalic acid, isophthalicacid, benzoic acid, p-carboxyphenyl acetic acid, p-p-dicarboxyldiphenylacid, p-t-butylbenzoic acid. Examples of cycloaliphaitc acids arehexahydrobenzoic acid, cis and transhexahydroterephthalic acid,hexahydrosalicylic acid, 2,5- dichlorocyclohexane-dicarboxylic acid-1,4;2-chlorocyclohexane-dicarboxylic acid 1,4;4-chlorocyclohexanedicarboxylic acid-1,3;4-chloro-l-carboxy-cyclohexane.

In the absence of indications to the contrary, all proportions andconcentrations of materials are expressed herein on a weight basis.

The invention may additionally be described by the following examples:

EXAMPLE I Solubility in water Pure terephthalic acid was mixed withwater and tested for solubility. Results indicated the acid was about0.001% soluble at 25 C. and 0.03% soluble at 100 C. Practicalrecrystallization of terephthalic acid required a minimum of 251%solubility which was not attained in water until a temperature of 280 C.was reached. In sharp contrast, in a 2 M lithium acetate solutionterephthalic acid reached 25% solubility at a temperature of 160 C.

Solubility in salt solution 50 g. of terephthalic acid was mixed with a250 ml. of 2. molar aqueous solution of lithium acetate at a convenientlaboratory temperature of 100 C. Tests showed the terephthalic acid tobe 6.8% soluble in the salt solution. Recrystallization was possiblefrom a solution saturated at 100 C. by reducing the temperature to 25 C.Crystals were filtered, washed with water, and dried for an hour at 110C.

4 Analysis showed the needle-like crystals obtained to be 100%terephthalic acid and no evidence of lithium was found.

The solubility of terephthalic acid was found not to be in proportion tothe concentration of the salt used as indicated by the following table.

TABLE I.EFFECT 0F SALT CONCENTRATION 0N TEREPHTHALIC ACID SOLUBILITYSolubility, g. TPAllOO g. solvent EXAMPLE H 250 ml. of 2 M solution oftriethylammonium acetate (32.2%) in water was intimately mixed with 50g. of terephthalic acid. Conventional solubility tests indicated thatthe acid was 4% soluble in the aqueous solution.

EXAMPLE III 50 g. of trans-hexahydroterephthalic acid was mixed with 250ml. of water and tested for solubility by conventional means at 25 C.and 100 C. The test was repeated, at 20 C. and 100 C., using the sameproportions, with a l M lithium acetate aqueous solution beingsubstituted for the water of the first experiment. The results were asfollows:

SOLUBILITY TO TRANS-HEXAHYDROTEREPHTHALIO ACID IN WATER AND 2 M LITHIUMACETATE SOLUTION Percent Temperature--..;.---;;-;:.:.:.;.':.;-. 20 C. 25C. 100 C.

Trans-hexahydrotereplzthalic acid in water 0. 1 1. 3Trans-hexal1ydroterephthalic acid in 2 M lithtum acetate solution 1.76.0

EXAMPLE 1V Experiments were conducted using 50 g. samples ofterephthaldehydic acid and p-toluic acid. The samples were mixed with250 ml. of water and a 2 M lithium acetate solution, respectively. Theresults of the conventional solubility tests are shown below:

SOLUBILITY OF TEREPHTHALDEHYDIG AND P-TOLUIC ACIDS IN WATER AND LITHIUMACETATE SOLUTION Percent Temperature--..e.:;::-;:::;.:;.-;...;:;.:.:.-20 C. 40 C. C:

Terephthaldehydic acid in water 0. 015 O. 075 0. 13 Terephthaldehydicacid in lithium acetate solution 8. 00 10. 60 p-Toluic acid in water 0.035 0. 08 0. 45 p-Toluic acid in lithium acetate solution 3.00 3. 58 7.8

EXAMPLE V SOLUBILITY OF TEREPHIHALIC ACID IN AQUEOUS 2 M SALT SOLUTIONS25 product crystal Solubility, analysis gJlOO g. solas TPA, Salt vent at100 percent Ammonium acetate 3. 58 Sodium acetate 2.0 45 Potassiumacetate- 1. 6 42 Lithium acetaten 6. 8 100 Rubidium acetate 2.0 44Cesium acetate 0. 8 40 Tiimethylammonium acetat 3.0 100 Tnethanolammomumacetate 7. 0 100 Lithium perchlorate 0 Lithium chloride 0.1 Lithiumformate... 1. 8 100 Lithium propionat 8. 8 100 Lithium succinate..- 7. 4100 Sodium acid succinat 0.3 70 Triethanolarmnonium acid oxalate 0. 2100 Lithium terephthalate (0.8 M) 0.8 100 Triethylammonium acetate 4. 0100 Sodium methanesulfonate. 0 Sodium toluenesulionate Triethylammoniumchloride Lithium aminoacetate 1 No crystals.

EXAMPLE VI Tests were conducted using a 2 molar lithium acetate solutionto which was added varying percentages of ethanol or acetic acid ascoupling agents. The resulting solubility data tabulated below show thatthe solubility of the organic acids in aqueous salt solution can begreatly affected by the addition of such coupling agents.

EFFECT OF COUPLING AGENTS ON SOLUBILITY OF SALT SOLUTION Solvent, 2 Mlithium acetate Grams/100 g. solvent Ethanol, Temp., Terephth. 1120,percent percent C. acid O-toluic Acetic acid, percent The principle,preferred embodiments, and modes of operation of the present inventionhave been described in the foregoing specification. However, it shouldbe understood that the invention which is intended to be protectedherein may be practiced otherwise than as described without departingfrom the appended claims.

We claim:

1. A process for treating a relatively insoluble organic carboxylic acidselected from the group consisting of phthalic acid, terephthalic acid,isophthalic acid, benzoic acid, p-carboxyphenyl acetic acid,p-p-dicarboxyl-diphenyl acid, p-t-butylbenzoic acid, hexahydrobenzoicacid, cis and trans-hexahydroterephthalic acid, hexahydrosalicylic acid,2,5-dichlorocyclohexane-dicarboxylic acid- 1,4;2-chlorocyclohexane-dicarboxylic acid-1,4;4-chlorocyclohexane-dicarboxylic acid-1,3; and4-chloro-1-carboxy-cyclohexane, which acid contains an aldehyde impurityand recovering said carboxylic acid in crystalline form of increasedimpurity, which process comprises mixing said carboxylic acid with anaqueous solution with a salt of a weak acid, said weak acid having a pKvalue of from 3 to 5, said salt having a cation taken from the groupconsisting of (a) lithium,

(b) a tertiary ammonium ion corresponding to the formula:

in which R R and R may be alkyl, aryl or alkylaryl, and (c) quaternaryammonium ion of the formula (R).;N+ where each R represents the same ordifferent alkyls of from 1 to 6 carbon atoms, heating the mixture to atemperature of between about 70 and 200 C. to dissolve said carboxylicacid, treating the resulting solution with a reagent to convert saidimpurity to a material which can be separated from said acid uponrecrystallization, cooling said solution to a temperature sufficientlylow to crystallize the carboxylic acid from said solution, andseparating the thus crystallized carboxylic acid from the solution,which acid contains less of said impurity than originally contained insaid carboxylic acid.

2. A process according to claim 1 wherein said weak acid salt has as acation a tertiary ammonium ion cor-- responding to the formula in whichR R and R may be alkyl, aryl, or alkylaryl.

3. A process according to claim 1 wherein said carboxylic acid isterephthalic acid containing carboxybenzaldehyde as an impurity, saidweak acid salt is lithium acetate, and said treating is accomplishedwith an oxidizing agent to convert said aldehyde to the correspondingacid.

4. The process of claim 1 wherein said salt is lithium acetate.

5. The process of claim 1 where-in said salt is triethylammoniumacetate.

6. The process of claim 1 wherein said carboxylic acid istranshexahydroterephthalic acid.

7. The process of claim 1 wherein said carboxylic acid is terephthalicacid.

8. The process of claim 1 wherein said treating is accomplished with anoxidizing agent.

9. The process of claim 8 wherein said carboxylic acid is terephthalicacid containing p-carboxybenzaldehyde impurities 10. The process ofclaim 9 wherein said weak acid salt is lithium acetate.

11. The process of claim 10 wherein the aqueous lithium acetate solutioncontains between about 5 and 70% of an organic coupling solvent takenfrom the group consisting of water miscible alcohols of from 1 to 5carbon atoms, acetone and methyl ethyl ketone.

References Cited UNITED STATES PATENTS 3,115,521 12/1963 Swakon 260525FOREIGN PATENTS 23,401 10/1964 Japan 260-525 723,680 12/196'5 Canada260525 OTHER REFERENCES Tipson: Crystallization and Recrystallization,in Technique of Organic Chemistry, vol. III, 1956, pp. 472-3.

LORRAINE A. WEINBERGER, Primary Examiner R. S. WEISSBERG, AssistantExaminer U.S. Cl. X.R.

260-5l4 R, 514 J, 525

